1. Field of the Invention
This invention relates to an apparatus for automatically aligning two different objects with each other. More specifically, it relates to an apparatus which employs a source of coherent light as the light source for alignment and in which predetermined reference marks are formed on two different objects and when the two objects are to be mutually aligned, diffracted lights from the alignment marks on the objects illuminated by the coherent light are caught as the optical position signals of the alignment marks and further, the optical position signals from the alignment marks on the two different objects are detected entirely independently so that the objects are displaced in response to the optical position signals, thereby accomplishing automatic mutual alignment of the objects.
2. Description of the Prior Art
Applications of the apparatus for automatically aligning two different objects with each other have been considered in many fields. Especially, in the process of forming an integrated circuit pattern on a semiconductor wafer by using more than two different photomasks in a semiconductor printing apparatus, an important task for enhancing the efficiency and accuracy of the alignment work is to automatically align the photomasks and the circuit pattern on the semiconductor wafer in a predetermined relationship prior to the printing.
Therefore, the present invention will hereinafter be described by taking as an example the automatic alignment apparatus in the semiconductor printing machine, whereas the invention is not restricted thereto but is applicable in a wider field wherein alignment is required.
The conventional automatic alignment apparatus in the semiconductor printing machine will be described with respect to one which employs a photoelectric microscope as shown in FIG. 1 of the accompanying drawings.
Designated by 1 is a photomask and denoted by 2 is a semiconductor water coated with a sensitive agent such as photoresist.
By an image forming lens 3, the image of an alignment mark formed on the semiconductor wafer 2 is formed on a vibratory slit 5, and the light beam passed through the vibratory slit 5 is converted into an electrical signal by a photoelectric converter element 6. A signal from an oscillator 7 for driving the vibratory slit 5 is applied as a reference signal to a synchronism detector 8. The signal applied from the photoelectric converter element 6 to the synchronism detector is synchronism-detected and thence passed through a power amplifier 9 to drive a servomoter 10 which controls a deflector 4.
Now, if the image of the alignment mark on the semiconductor wafer 2 does not lie at the center of vibration of the vibratory slit 5, the output of the synchronism detector 8 will be a DC signal corresponding to the positional deviation of the alignment mark on the semiconductor wafer. In accordance with this DC signal, the servomotor 10 drives the deflector 4, whereby the beam of image light from the alignment mark on the semiconductor wafer 2 is deflected so as to effect automatic control such that the image of the alignment mark on the semiconductor wafer 2 lies at the center of vibration of the vibratory slit 5. Thus, positioning of the semiconductor wafer 2 is effected. In order that such operation may be accomplished, the alignment mark on the photomask 1 must be at a position offset from the view field of the image forming lens 3. This is because if the alignment mark on the photomask 1 should lie within the view field of the image forming lens 3, this alignment mark cannot be discriminated from the alignment mark on the semiconductor wafer 2.
When the positioning of the semiconductor wafer 2 has been completed, the deflector 4 is fixed intactly, and then positioning of the photomask 1 is carried out in the same manner as described with respect to the semiconductor wafer 2. More specifically, the photomask 1 is displaced in a predetermined direction by a servomotor 11 so that the alignment mark on the photomask comes into the view field of the image forming lens 3. The image of the alignment mark on the photomask 1 is formed on the vibratory slit 5 and if this image is deviated from the center of vibration of the slit, an error signal is produced by the synchronism detector 8 to drive the servomotor 11, whereby the positioning of the photomask 1 is accomplished.
Such a positioning method using a photoelectric microscope has the following two disadvantages:
(A) If positioning of the photomask 1 is effected after the positioning of the semiconductor wafer 2, the images of the alignment marks on the photomask and the semiconductor wafer overlap each other on the vibratory slit 5. Ultimately, the alignment between the images of these two alignment marks is effected by the positioning of a composite optical image comprising the images of the two alignment marks. If the composite optical image has an error with respect to the alignment between the semiconductor wafer and the photomask, such error will directly affect the accuracy of the positioning.
(B) There is a problem of contrast of the alignment mark images. Especially, the semiconductor wafer is subjected to several steps of necessary treatment during the manufacture of integrated circuit and under the influence of such treatment, the alignment mark on the semiconductor wafer may be greatly reduced in contrast as an optical image. Reduced contrast of the alignment mark affects the accuracy with which the position detection is effected by the photoelectric microscope.
These two disadvantages (A) and (B) would offer essential problems not only in the alignment apparatus using the photoelectric microscope but also in the alignment apparatus utilizing the optical images of the semiconductor wafer and the photomask.